Polyurethane compositions have been used as sealants or adhesives for many years. Polyurethane adhesives or sealants in general have relatively high bond strengths, flexibility, shock and impact resistance, fatigue resistance, and the like. This set of highly desirable properties primarily arises from a crosslinking reaction, known as curing, which is designed to occur once the adhesive or sealant is applied. As the adhesive or sealant cures, it transforms from a soft, workable liquid or semi-liquid to a firm, resilient, elastomeric solid. It is to be understood that hereinafter, the term adhesive will be used in the identification of the polyurethane composition of the present invention, and is meant to encompass the term sealant or any other term which might be used in identifying such polyurethane compositions as are described immediately above.
Polymeric adhesives therefore are sold in a non-cured form containing polymerizable components which are designed to cure upon application. Many such adhesives are created from a prepolymer component and a curative component. Typically for polyurethane compositions, the prepolymer component is the reaction product of a polyol and an organic polyisocyanate having isocyanate functional groups in a weight percent amount of from about 0.5% to about 10%, and more typically from about 1% to about 4% based on the weight of the prepolymer. The curative generally is a reactive hydrogen-containing material.
The prepolymer and curative components of the polyurethane adhesive composition can be packaged separately creating so-called two-part or two-step products. Two-step systems, which posses many desirable characteristics, generally require mixing just prior to application, and such mixing is often found to be inconvenient and problematic. Therefore, one-step systems containing all components of a polyurethane adhesive composition in a single package are preferred by many end users. Such one-step polyurethane compositions generally will cure due to ambient conditions, and most often due to moisture in the air. In polyurethane systems, ambient moisture can react with the isocyanate reactive sites and cause polymerization directly or can react with an intermediary, such as oxazolidine or ketimine, to produce a reaction product (typically an amine) which, in turn, causes curing. Even if the system is the one-part type, the adhesive composition usually also will contain sufficient urethane catalyst to produce a desired cure time. The above-described one- and two-part polyurethane adhesives are widely employed in many industries, including the motor vehicle and construction industries.
For application to vertically-oriented areas, such as wall surfaces or window frames, an adhesive material is required to have non-sag characteristics, i.e. be substantially free from or resistant to sagging or slumping after application. A non-sag type adhesive material should flow easily when subjected to external forces during its mixing and application, but should be free from flow when it is at a standstill and therefore should exhibit a substantial increase in apparent viscosity. Such behaviour is generally described as thixotropic or yield stress related. It is to be understood that for purposes of further discussion hereinbelow, the term "sag" encompasses two types of sag identified by those having ordinary skill in the art: (i) immediate sag which generally manifests itself immediately upon manufacture of the material, and (ii) latent sag, which develops during storage of the material.
Sag-resistant polyurethane adhesives generally are known in the art, but can have drawbacks. Such materials require a precisely optimized formulation which may be unforgiving if any errors occur during the manufacturing process. Moreover, these materials are sometimes also sensitive to the storage and curing temperatures which can adversely affect the anti-sag properties and the mechanical properties of the cured product.
Heretofore, the known methods of enhancing the non-sag characteristics in polyurethane adhesive-products included:
1. the use of special sag-resistant additives, usually castor oil derivatives; PA0 2. the utilization of highly structured fillers with high surface areas which tended to agglomerate, such as different grades of carbon black and amorphous silica; PA0 3. the use of high concentrations of certain fillers or filler combinations such as special grades of calcium carbonate; or PA0 4. the utilization of swellable polymers such as polyvinyl chloride. PA0 1. Castor oil derivatives and other commercially available non-sag additives are reactive with isocyanate groups, and therefore, the polyurethane products formulated therewith have limited storage stability which has a detrimental effect on their practical usefulness. PA0 2. Amorphous silica causes serious shrinkage problems during cure of the polyurethane product and frequently causes adhesion problems. PA0 3. Polyvinyl chloride requires a very precise manufacturing process and the final polyurethane product usually has limited ultraviolet light (UV) stability. PA0 4. Carbon black can be used for manufacturing of black polyurethane products only. PA0 5. In order to achieve the required non-sag properties, the concentration of carbon black or other fillers must be close to that of the critical pigment volume concentration value, and this may create some difficulties in manufacturing thereby adversely affecting the final mechanical properties of the polyurethane product.
Each of these non-sag enhancement systems of the known prior art have serious disadvantages:
Urethane materials known in the art generally comprise in addition to the urethane prepolymer, curative catalyst and conventional non-sag additive as described above, a plasticizer and/or solvent, fillers, an adhesion improver, pigment(s), UV stabilizers, and a moisture scavenger. The fillers contribute to increasing the volume of sealant material, and also to adjusting the mechanical properties such as hardness and tensile properties.
In particular, polyurethane adhesive compositions lacking fillers generally exhibit low sag resistance, and therefore fillers have generally routinely been incorporated therein to develop or increase sag resistance where such resistance is needed. Unfortunately, this technique also increases the viscosity of the component or components containing the filler. Therefore, loading with high levels of filler partially increases sag resistance, but typically results in difficulties in obtaining satisfactory mixing because shear mixing equipment and high pressure pumping equipment generally are needed. Furthermore, high filler loading in many cases tends to lower the strength of the adhesive bond and typically decreases elasticity. The increased viscosity typically makes the adhesive more difficult to apply.
As discussed hereinabove, polyurethane adhesives display many highly desirable properties primarily due to the crosslinking reaction or curing which takes place once the adhesive is applied. However, the mechanical properties of the cured adhesive are directly related to curing conditions. The mechanical properties of many known prior art polyurethane adhesive formulations are adversely affected by changes in ambient conditions during curing.
Conventional compounding techniques make it possible to vary the elastomeric properties and flow properties of a polyurethane adhesive composition, but it generally has been very difficult to achieve the desired combination of manufacturing ease, extrudability, non-sag properties, and good mechanical properties merely by judicious choice of compounding conventional ingredients.
Thus, the need exists for a moisture curable one-part polyurethane adhesive composition having good stability in the absence of moisture and a generally rapid cure rate in the presence of atmospheric moisture, and which is sag-resistant and exhibits suitable mechanical properties after cure despite variations in ambient conditions during application and curing of the adhesive.